Vapor temperature control method



Aug. 24, 1965 w, w. SCHROEDTER 3,202,135

VAPOR TEMPERATURE CONTROL METHOD Original Filed July 27, 1961 2 Sheets-Sheet 2 WITHCUT RECIRCULATION WITH RECIRCULATION INVENTOR. WILLBURT W. SCHROEDTER WORKING FLUID TEMPERATURE F FIG. 6

ATTORNEY the furnace walls. system is a circulating system which is effective to circulate fluid through the furnace wall tubes or at leastthe the medium in the tubes.

3,202,135 VAPOR TEMPERATURE CONTRGL METl-IOD Willhurt W. Schroedter, West Hartford, Conn, asslgnor to Combustion Engineering, Inc, Windsor, Cnn., a corporation of Delaware Original application July 27, 1961, Ser. No. 127,176. Divided and this application Dec. 17, 1%3, Ser. No.

4 Claims. (Cl. 122-406) tubes disposed along its walls which form part of the through-flow system with combustion gases passing longitudinally of the furnace and through the furnace outlet with these gases passing over convection heat absorbing surface wherein the working medium which is supplied to a turbine is reheated to a predetermined desired temperature after a portion of its energy has been utilized in the turbine.

In accordance with the invention the heat absorption within the furnace is varied by means of introducing recirculated combustion gases thereinto and simultaneously recirculating fluid through the furnace Wall tubes so as to vary the heat content of the gases passing over the convection heat absorbing surface in a manner which will tend to, at least in part, offset the effect of variation of load onthe reheat vapor temperature.

The supercritical vapor generator with which the invention is concerned has the walls of the furnace lined with fluid heating tubes which form part of the through-flow circuit of the generator. The arrangement is such that a header is provided in the lower region or lower end of the furnace with which tubes which extend. upwardly therefrom are connected. These tubes effectively line Superimposed upon the through-flow furnace wall tubes that line the lower region of the furnace. This recirculation of the working] medium through the furnace wall tubes is effective in such a manner that below a predetermined load on the unit the recirculationis initiated and as the load decreases below this predetermined load the proportion of the recirculated fluid to the total flow through the tubes is progressively increased. The effect of this recirculation of the fluid through the furnace wall tubes is to increase the temperature of the working medium entering these tubes and decrease the difference in temperature of the working medium between the inlet and outlet of the tubes with there being an increase in the average temperature of This in turn will decrease the convection and radiant heat pickup of the furnace wall tubes or more correctly the working medium upon travcrsal of these tubes for a given firing rate because the heat head, i.e. the difference in temperature between the heated and the heating medium, is decreased. Simultaneously with this recirculation of working medium through the furnace wall tubes and which recirculation is increased with decreasing load there is introduced into the. furnace recirculated combustion gases which have traversed the convection reheating surface of the unit and with these gases being introduced into the furnace United States Patent 0 ice in such a manner that the heat pickup of the medium upon traversing the furnace wall tubes is decreased. Accordingly this simultaneous recirculation of fluid through the furnace wall tubes and the recirbulation of combustion gases work together to increase the heat content of the gases passing through and out of the furnace. reheating portion of the supercritical vapor genera-tor 1s predominately convection surface disposed in the path of these gases. Such a unit has the characteristic of having the temperature of the reheat vapor tend to decrease with decreasing load on the unit. This tendency may be offset at least in part, if not wholly, throughout the desired load range of operation of the unit by the previously mentioned steps whereby the heat absorption in the furnace is decreased and the heat content of the gases egressing therefrom and traversing the convection reheating portion is increased. Accordingly as the load on the unit is decreased the recirculation of the working medium through the furnace wall tubes is increased and the amount of gas recirculation introduced into the furnace is increased thereby making more heat available to the reheater for any particular firing rate and. compensating at least in part for the tendency of the reheat vapor to decrease with decrease in load.

it is noted that by employing recirculation of the working medium together with gas recirculation, the operating characteristic within the through flow circuit, and particularly the furnace walls, is improved with there being greater stability and with there being more uniform temperature of the tubes in addition to effecting the desired and relatively substantial control of the reheat vapor temperature.

Accordingly it is an object of the present invention to provide an improved control arrangement and scheme for forced through-low vapor genera-tors operating at supercritical pressure on the reheat cycle and wherein the reheat is effected predominately by convection heat exchange.

A still further object of the invention is to provide such an improved control wherein the heat absorption in the furnace is varied with varying load both by means of gas recirculation and by means of recirculation fluid to the tubular walls lining the furnace.

The invention will be more fully understood from the following description when considered in conjunction with the accompanying drawings forming a part thereof and in which;

FIG. 1 is a vertical section through a high capacity vapor generator operating at supercritical pressure and organized for carrying out the improved method of the invention with this illustration being somewhat diagrammatical in nature;

FIG. 2 is a transverse sectional view through the furnace of FIG. 1 being taken generally along line 2-2 of FIG. 1;

FIG. 3 is a view similar to that of FIG. 1 showing only the lower portion of the furnace and illustrating a modified construction relative to that of FIG. 1;

FIG. 4 is a view similar to that of FIG. 3 showing still another modified construction;

FIG. 5 is a sectional view taken 5-5 of FIG. 4; and

FIG. 6 is a graphic representation illustrating the fluid generally along line temperature rise in the furnace walls at a particular load on the generator organizations of FIGS. 1 and 4.

Referring now to the drawings, wherein like reference characters are used throughout to designate like elements, the illustrative organization depicted therein includes a supercritical vapor generator having a furnace which is generally designated 19 and upwardly through which combustion gases pass with the furnace outlet being located at the upper region of the furnace and with lateral The temperature.

3 gas pass 12 extending from this outlet and being connected to the upper end of gas pass 14. This latter gas pass extends downwardly in generally parallel relation with the furnace and connects, through duct 16, with a suitable air heater or the like which in turn is connected to a stack.

The through-flow circuit of the supercritical generator includes economizer 18 positioned in the lower region of gas pass 14-. This economizer, which is preferably comprised of sinuously bent tubes, receives the through-flow at supercritical pressure from feed pump Ztl and after traversing the economizer the working medium passes upwardly to the distribution header 22. From this distribution header the working medium is conveyed through conduit 24 to mixing vessel 26 and then from this mixing vessel through conduit 28 to the distribution header 3%). From this header the fluid is distributed to the furnace wall tubes 32 which extend upwardly along all four of the furnace walls, lining the inner surface thereof, with the tubes being in side-by-side relation and in parallel fiow I relation with respect to the working medium so that upfiow of the fluid medium though these tubes is provided. As illustratively disclosed in FIG. 1 the tubes 32 which line the lower region of the furnace extend from header 3% to sub headers 3d and then continue on from these sub-headers up the remaining vertical extent of the furnace Walls with these tubes eventually terminating in header 36. From this header 36 the through flow is conveyed through conduits 38 to the header 40 and from this header 4'0 the working medium flows through the tube bundle or heat exchange section 42 being discharged from this section into header 44. From header 44 the working fluid is conveyed through conduit 46 to header 48 and from this header through the heat exchange tubular panels with the vapor from these panels being collected in header 52 and then conveyed through conduit 54 to the turbine 56.. Y

Disposed within the lateral gas pass 12 is the reheater 5% which is predominately convection heating surface and is comprised of a series of rows of tubes disposed in side-by-side relation across the gas pass with this reheater receiving vapor from the turbine through the conduit 60 and with the reheater bein effective to reheat the vapor to the desired temperature, with this vapor then being reintroduced into the turbine via conduit 62.

The discharge from this turbine is received by condenser 64 which is effective to condense the vapor and the condensate is pumped by condensate pump 66 through the feed water header 68, deaerator 7d and then to the feed pump Since the vapor generator is operated at supercritical pressure, as for example 3500 lbs. per sq. in. delivered to the turbine, the working medium will continuously rise in temperature as it traverses the through-flow circuit. Accordingly as the working fiuid passes from header up through the wall tubes 32 it progressively increases in For example, at a given load such as 30 percent load the temperature of the medium entering header 30 may be 540 while that discharged to header 36 may be 750.

It is a characteristic of vapor generators to which the -invention is directed that the reheat vapor delivered to the turbine has a tendency to fall in temperature as the load on the generator is decreased. Since it is necessary that this reheat temperature delivered to the turbine be maintained constant throughout the operating load range the reheat vapor temperature to decrease in load is overcome at least in part by means of introducing recirculated gases into the furnace and by means of simultaneously recirculating fluid medium through the furnace wall tubes. The combination of these actions provides a corrective efiect that has a substantial influence on the vapor temperature over a relatively wide load range.

in the illustrative organization of FIG. 1 furnace 10 is fired by means of cyclones 72 which may be constructed and operated in the manner disclosed by US. Patent 2,594,312 to Kerr et al. and/or US. Patent 2,357,301 to Bailey et al. Hot combustion gases exit from these cyclones into the lower region of furnace 1t These gases which are at a high temperature pass up through furnace lift and then over the heat exchange portions 50, 58, 42 and 18 with the gases finally exiting from the lower end of gas pass 14-. Within furnace 19 heat is imparted to the fluid passing up through the tubes 32 lining the furnace walls with the greatest heat intensity in the furnace being at the lower region thereof.

In lieu of the cyclone type of firing, the furnace may be fired by any one of a variety of other known firing arrangements with FIGS. 3 and 4 illustrating two such known arrangements. In the FIG. 3 illustration the front wall firing method is shown wherein the burners 73 are arranged to project fuel into the furnace from the front wall and with the fuel being burned in the lower furnace region.

FIG. 4 illustrates the tangential firing method wherein fuel and air are directed from the furnace walls through burners 75 tangent to an imaginary centrally disposed vertical cylinder and with the fuel again being burned in the lower region of the furnace.

Regardless of the firing method, the heat absorbed by the fluid passing up through wall tubes 32, for any given firing rate, may be varied and accordingly the heat content of the gases leaving the furnace may be regulated by means of reintroducing combustion gases into the furnace. These combustion gases, in accordance with the invention, are introduced into the lower region of the furnace in a manner which decreases the heat absorption of the furnace walls relative to that which would prevail without the recirculated gas. As illustrated in FIG. 1 the gas is taken from the lower end of duct 14 by means of the recirculating fan 76 and is introduced by this fan and distributing duct 78 into the lower region of the furnace through the discharge passageways 80. In the FIG. 3 illustration the recirculated gases are discharged through duct 78 into the hopper portion of the furnace while in the FIG. 4 illustration the recirculated gases are discharged through duct 78" so that they enter the furnace along and in conjunction with the fuel and air through the tangentially arranged burners '75. Regardless of which of the known types of firing systems is employed the recirculated combustion gases are introduced into the furnace in such a manner as to decrease heat absorption of the tubes that line the furnace walls resulting in increasing the heat content of the gases passing over the reheat heat exchange surface 58 and the other heat exchange surface downstream thereof so these heat exchangers reccive more heat than they would otherwise.

Simultaneous with the recirculation of combustion gases, there is provided a recirculation of the working medium through the furnace wall tubes or at least through the furnace wall tubes lining the lower region of the furnace with this recirculation of the working medium also being effective to decrease the heat absorption in the furnace. This is accomplished by providing a recirculating system super-imposed on the through-flow system and which is connected with the through-flow system at a point downstream of the furnace walls relative to flow of the working medium and to a point upstream of the furnace walls, with a suitable pump means being provided to effect recirculation through this system. In the illustrative organization of FIG. 1 there is connected with the outlet header 36, which is the collecting header for the parallel tubes lining the four furnace walls, conduit 82 which extends to and is connected with pump means 84.

Connected with the outlet of this pump means is conduit 86 which in turn is connected with the mixing vessel 26, with check valve 88 being provided in this conduit in order to prevent reverse flow from the mixing vessel to and through the pump 84. Mixing vessel 26 is elfective to mix the hot fluid pumped by pump 84 from header 36 with the relatively cold through-flow coming from economizer 18. It will be seen that this recirculating system is so arranged that a portion of the working medium may be withdrawn from the header 36 and returned via conduit 82, pump 84, conduit 86, mixing vessel 26 and conduit 28 to the inlet header 30 for repassage up through the parallel tubes 32 lining the furnace walls.

The effect of recirculating a portion of the working medium through the parallel furnace wall tubes 32 is to increase the average temperature of the medium in these tubes with there being a substantial increase in the temperature of the medium at the inlet or the lower end of the tubes over that which would prevail without recirculation, and with the temperature rise of the medium, due to recirculation, becoming progressively smaller as the flow progresses up through the tubes. This effect is illustrated in FIG. 6 which is a graphic representation at one load of the unit of the temperature of the fluid medium passing through the furnace wall tubes 32 both with and without recirculation. The dotted line curve 90 represents the temperature of the working fluid passing up through the furnace wall tubes Without recirculation, while the solid line curve 92 represents this temperature with recirculation with both of these curves indicating this temperature at a particular load, such as 30 percent load, since the temperature will change somewhat as the load changes. From these FIG. 6 curves it will be seen that by means of recirculating a portion of the working medium through the furnace wall tubes the temperature of the fluid entering these tubes may be raised substantially and of course the average temperature throughout the length of the tubes will be raised with the greatest elfective rise in temperature being provided in the lower region of the furnace. This is the region Where the heat in the furnace is most intense and by means of this recirculation the convective and radiant heat transfer to the medium flowing through the tubes may be varied since the heat head or temperature differential between the heating and the heated medium will be varied by this recirculation. By raising the temperature of the working medium passing through the tubes and particularly at the lower region of the tubes 32 the temperature difference between the heating and the heated medium will be decreased so that the amount of heat transferred to the fluid flowing through tubes 32 will be decreased resulting in the gases passing over the superheat and reheat heat exchange surfaces having a greater heat content.

As previously mentioned the vapor generator with which the invention is concerned has the characteristic of having the reheat vapor temperature tend to fall with decreasing load on the unit. This tendency is overcome at least in part in accordance with the present invention by means of simultaneously recirculating combustion gases as described herein before and recirculating fluid medium through the furnace wall tubes as previously described. The control arrangment is such that as the load on the unit decreases, for instance, below a predetermined load such as 90 percent of full load, recirculated gases are controllably introduced into the furnace and a portion of the working medium is recirculated through the furnace wall tubes. This results for a given firing rate in decreasing the heat absorption in the furnace and making a greater amount of heat available in the combustion gases passing over the heat exchange surface disposed in the combustion gas stream and accordingly imparting more heat to the fluid flowing through these surfaces thereby raising the temperature of the reheat vapor over what it would otherwise be. Since the tendency of the reheat vapor temperature to fall increases as the load decreases the amount of recirculated gases introduced into the furnace is increased with decreasing load and the amount of the working medium recirculated through the furnace wall tubes increases with decreasing load so that the proportion of recirculated fluid to the total flow through the furnace wall tubes increases with decreasing load. The combination of recirculating the working medium through the furnace Wall tubes and introducing recirculated combustion gases into the furnace, in addi tion to providing a substantial control effect by which the reheat temperature may be regulated with decreasing load, also improves the operating characteristics of the unit over those which would prevail without this particular combination of steps. The introduction of recirculated gases in the manner described decreases the heat transfer that takes place in the furnace with this effect being greater in the region of the furnace adjacent the firing thereof and decreasing from this region toward the furnace outlet. Therefore, with this gas recirculation proportionately-more heat is absorbed by the furnace wall tubes in the region remote from the firing. In the organization of applicants invention this is the region of these tubes where the fluid medium is the hottest. The recirculation of the fluid medium increases the fluid flow through these tubes insuring adequate flow which is of particular importance in this hottest region and also insures stability within the various parallel flow paths so as to provide more uniform tube temperatures and obtain a better operating condition.

While the illustrative organization provides for recirculation throughout the entire furnace height it may be advantageous to recirculate the working medium only through the lower furnace region, with this being the region of most intense heat as previously mentioned. This may be achieved by connecting the inlet of the circulating system with headers 34 or some other suitable header arrangement may be provided. Furthennore it is not necessary to arrange furnace wall tubes 32, so that the fluid medium flows in parallel flow relation only in an upper direction through these tubes. The benefit of reduced heat absorption by recirculation of the working medium may be provided by having tubes 32 extend up and down one or more times along the furnace wall or along the lower portion of the furnace wall. However the optimum effect is obtained with the parallel flow arrangement wherein each of the sideby-side tubes 32 is a single upwardly extending tube connected between the inlet and outlet headers.

In operation of units of the type to which the invention pertains, i.e. supercritica l units which are provided with through-flow circuits, the temperature of the primary vapor is controlled by the firing of the unit and the pressure is controlled by the feed or through-flow pump. A spray attempera-tor 94 may be incorporated intermedi ate the heat exchange port-ion 42 and the final or finishing heat exchange portion 50 of the unit in order to provide more sensitive and accurate control and to prevent overriunning of the primary vapor temperature. However the principal control for this primary vapor temperature is achieved by controlling the firing of the unit with the firing being decreased to decrease the primary vapor temperature and increased to increase the same. The reheat vapor temperature is controlled through the coirtrol actions previously described. The reheat vapor temperature delivered to the tunbine may be sensed, such as at the location 85, and the indication thus achieved may be employed through known control organizat-ions to obtain adjustment or regulation of the perature balance, will be improved over what they would otherwise be.

Accordingly it will :be appreciated that there is provided in accordance with the present invention an improved method Where, in a supercritical, forced throughflow vapor generator, the tendency of the reheat vapor temperature to rise with decreasing load is overcome at least in part by means of an improved method of operating such an organization.

It will be understood that the foregoing description is intended for the purpose of illustration only and that modifications such as will occur to those skilled in the art are possible and are embraced within the scope and spirit of the invention.

What I claim is:

1. In a forced througluflow vapor generator operating at supercritical pressure and on the reheat cycle, said generator including a furnace through which combustion gases are conveyed with the furnace walls being lined with tubes which form part of the through-flow system, said generator also including predominatively convective reheating surface over which said combustion gases are conveyed, the method of increasing the heat input to said vapor reheating surface over what it would otherwise be with decreasing load comprising introducing a portion of the combustion gases after traversal of the reheating surface into the furnace in a manner to effect a decrease of the heat absorption in the furnace, increasing this gas r circulation as the load decreases, simultaneously recirculating a portion of the Working medium through at least a portion of the through-flow system lining the furnace walls by withdrawing Working medium from the through-fiow system at a location downstream of the portion lining the furnace walls and introducing it into said system upstream of said portion with generally the entire heat content of this withdrawn fluid beingcontained in the fluid entering the portion of the circuit lining the furnace walls and increasing the proportion of the recirculating fluid to the total flow with decrease in load.

2. The method of generating vapor .at supercritical pressure and reheating the same after a portion of its energy has been dissipated comprising generating a combustion gas stream by the burning of a fuel, forcing the working medium at supercritical pressure through a continuous path and imparting heat generated by the burning fuel to said medium during its traversal of said path and in sufficient quantity to heat said medium to a desired value, during its conveyance along said path passing the Working medium in confined stream-s in bounding relation with said combustion gas stream and in a first heat transmission zone where heat is imparted from said combustion gas stream to said confined streams, utilizing a portion of the energy of the thus heated working medium and thereafter reheating the same 'by conveying the Working medium in heat exchange relation with the gas stream and imparting heat to said medium from the gas stream principally by convection and at a zone downstream of the first heat transmission zone, as the vapor generating rate decreases recirculating to the said first zone gases partially cooled in the convection zone and in a manner to decrease the heat absorption in said first heat transmission zone, and recirculating in said confined streams working medium heated in said first heat transmission zone by withdrawing Working medium from the continuous'path at a location downstream of said first heat transmission zone and introducing it into the continuous path upstream of said first heat transmission zone with generally the entire heat content of this withdrawn medium being contained in the medium being introduced upstream of said first heat transmission zone, and increasing the gas recirculation and the fluid recirculation with decreasing load,

3. The method of vapor generation comprising forcing the Working medium at supercritica-l pressure through a continuous path including confined parallel streams bounding a furnace and extending longitudinally thereof, generating a stream of combustion gases by burning a fuel and passing said gas stream through said furnace in parallel relation with said confined parallel streams while imparting heat thereto principally by radiation, additionally irnparting heat from the gas stream to the medium passing through said path in sufiioient quantity to produce vapor at the desired temperature, after utilizing a portion of the energy of this vapor reheating the same by passing it in convection heat exchange relation with said combustion gas stream, with decrease in the generation of vapor recirculating to the furnace combustion gases partially cooled by convectively reheating the vapor and in a manner to decrease radiant absorption by said confined streams and also recirculating working medium through said confined parallel streams by withdrawing a portion of the working medium from said path at a location downstream of said confined streams and introducing it into said path upstream of said confined streams with generally its entire heat content being contained in the introduced working medium, increasing this recirculation of gases and the Working medium with decreasing load.

4. The method comprising generating a combustion gas stream by burning a fuel, forcing a working fluid at supercritical pressure through a continuous path and imparting heat generated by the burning fuel to said fiuid during its traversal of said path and in suflicient quantity to heat said fluid to a desired temperature, during the conveyance of said fluid along said path passing the same in confined streams in bounding relation with the combustion gas stream in a radiant transmission zone Where the heat received is transferred predominately by radiation, conveying the working fiuid at said desired temperature to a point of use and utilizing a portion of the energy contained therein, thereafter reheating the working fluid by conveying the same in heat exchange relation with the gas stream and imparting heat to said fluid from said gas stream principally by convection and at a zone down stream of the radiant transmission zone, and simultaneously with a decrease in the fuel that is burned recirculating working fiuid in said confined streams disposed about said radiant transmission zone by withdrawing a portion of the Working fluid from said path at a location downstream of said confined streams and introducing it into said path upstream of said confined streams with generally its entire heat content being contained in the introduced working fluid and introducing partially cooled combustion gases into the gas stream at a location and in a manner so that the heat transfer to the confined streams in the radiant transmission zone is decreased, increasing the fluid recirculation and the gas recirculation with decrease of the fuel burned thereby increasing the heat content of the gases traversing said zone where the fluid is reheated principally by convection and thereby increasing the heat imparted to said fluid at said zone over that Which would be obtained Without such recirculation.

OTHER REFERENCES Combustion, of August 1956, pages 47 to 56, published by Combustion Publishing Co., Inc, of New York.

PERCY L. PATRICK, Primary Examiner.

KENNETH W. SPRAGUE, Examiner. 

1. IN A FORCED THROUGH-FLOW VAPOR GENERATOR OPERATING AT SUPERCRITICAL PRESSURE AND ON THE REHEAT CYCLE, SAID GENERATOR INCLUDING A FURNANCE THROUGH WHICH COMBUSTION GASES ARE CONVEYED WITH THE FURNACE WALLS BEING LINED WITH TUBES WHICH FORM PART OF THE THROUGH-FLOW SYSTEM, SAID GENERATOR ALSO INCLUDING PREDOMINATIVELY CONVECTIVE REHEATING SURFACE OVER WHICH SAID COMBUSTION GASES ARE CONVEYED, THE METHOD OF INCREASING THE HEAT INPUT TO SAID VAPOR REHEATING SURFACE OVER WHAT IT WOULD OTHERWISE BE WITH DECREASING LOAD COMPRISING INTRODUCING A PORTION OF THE COMBUSTION GASES AFTER TRAVERSAL OF THE REHEATING SURFACE INTO THE FURNACE IN A MANNER TO EFFECT A DECREASE OF THE HEAT ABSORPTION IN THE FURNACE, INCREASING THIS GAS RECIRCULATION AS THE LOAD DECREASES, SIMULTANEOUSLY RECIRCULATING A PORTION OF THE WORKING MEDIUM THROUGH AT LEAST A PORTION OF THE THROUGH-FLOW SYSTEM LINING THE FURNACE WALLS BY WITHDRAWING WORKING DOWNSTREAM OF THE THROUGH-FLOW SYSTEM AT A LOCATION DOWNSTREAM OF THE PORTION LINING THE FURNACE WALLS AND INTRODUCING IT INTO SAID SYSTEM UPSTREAM OF SAID PORTION WITH GENERALLY THE ENTIRE HEAT CONTENT OF THIS WITHDRAWN FLUID BEING CONTAINED IN THE FLUID ENTERING THE PORTION OF THE CIRCUIT LINING THE FURNACE WALLS AND INCREASING THE PROPORTION OF THE RECIRCULATING FLUID TO THE TOTAL FLOW WITH DECREASE IN LOAD. 